Electronic apparatus has become compact in shape and more and more sophisticated in the function. Semiconductor devices and other electronic components are requested to have a higher density and higher grade functions. In order to meet the recent technological needs, circuit boards are requested to be small in size, yet capable of having a number of electronic components in high mounting density. As a result, it has become an important task to efficiently dissipate the heat generated by densely mounted power semiconductor devices. A circuit board of improved heat dissipation is disclosed in Japanese Patent Laid-Open Application No. 2002-33558. A conventional method of manufacturing heat conductive substrate is described below with reference to the drawing. FIG. 14 is a cross sectional view showing the structure of conventional heat conductive substrate. Sheet-formed thermosetting resin composite 100 has been provided by mixing a thermosetting resin and a heat conductive filler. Lead frame 300 has been provided in the shape of a circuit pattern. Heat dissipating metal plate 500 and lead frame 300 have been integrated, with sheet-formed thermosetting resin composite 100 in between, to form a single body.
Electronic components 400 are mounted on lead frame 300.
However, the above described conventional method of manufacturing heat conductive substrate, in which a thermosetting resin composite material is heat-pressurized for integration, has following problems. Namely, the thermosetting resin composite material is softened and gets a fluidity during the heat-pressurizing process. As a result, the thermosetting resin composite is not only delivered to the space between the circuit patterns of lead frame 300 but it also oozes out on the surface of lead frame 300, or pushes up above.
If the composite material gets cured as it is in the oozed state or in the pushed-up state, the portion remains as stain on the wiring pattern or electrode pattern.
It leads to a problem of imperfect electrical or physical connection when electronic components are mounted on the substrate. As a countermeasure against the problem, viscosity during heat-pressurization of thermosetting resin composite 100 may be raised for controlling the fluidity. This may be effective to curtail the oozing and push-up of resin composite on the surface of lead frame 300; however, it generates new problems that the adhesive property between thermosetting resin composite 100 and lead frame 300 deteriorates, and the thermosetting resin composite readily generates pin-holes and bubbles.
The present invention offers a method of manufacturing heat conductive substrates, with which method the stain on the surface of lead frame 300 due to oozing or push-up of thermosetting resin composite 100 during heat-pressurization can be suppressed. The present invention also offers a method for providing electrically-isolated land portion with a high dimensional accuracy in an efficient way.